In semiconductor manufacture a wire bonding process is used to electrically connect bond pads formed on semiconductor dice to the lead fingers of a leadfree strip. Apparatus for wire bonding semiconductor dice are well known in the art. U.S. Pat. Nos. 3,894,671 to Kulicke Jr. et al; 4,877,173 to Fujimoro et al; and 5,082,165 to Ishizuka; disclose representative wire bonding apparatus.
In a conventional semiconductor packaging process, several semiconductor dice are attached to a leadfree strip. During the wire bonding process, each die is held between a wire bonding clamp and a wire bonding heat block of the wire bonding apparatus. The heat block heats the semiconductor die and the leadframe to a temperature of about 300.degree. C. to 400.degree. C. A bonding tool then mechanically presses a fine bond wire to a bond pad of the die and then to a bonding site on an appropriate lead finger.
The bond wire is typically a fine gold or copper wire that is threaded through the bonding tool. The end of the wire is heated by an electrical discharge or a hydrogen torch to a molten state. This forms a ball of molten metal on the end of the bond wire. For bonding a selected bond pad of the heated die, the molten ball is pressed by the bonding tool against the bond pad. This mechanically bonds the bond pad and the bond wire to one another. The bonding tool is then moved over a bonding site on a selected lead finger and the heated bond wire is pressed against the bond site. This mechanically bonds the bond wire and the lead finger to one another. The bond wire is then tensioned and sheared. This process is repeated for each bond pad on the die.
A critical requirement of the wire bonding process is precisely locating the bonding tool with respect to each bonding pad and to a bond site of an appropriate lead finger. Earlier versions of wire bonding apparatus were manually operated by an operator viewing the semiconductor dice through a microscope to precisely locate the bonding tool. More recently, automated wire bonding apparatus include vision systems for automatically sensing the locations of the bond pads on the dice and lead fingers of the leadframe to automatically perform the wire bonding process. U.S. Pat. No. 4,441,205 to Berkin et al. discloses a representative vision system for wire bonding apparatus.
In general, such vision systems are adapted to sense the lateral edges of the lead fingers and to teach a bond site in the middle of the lead finger somewhere along the longitudinal axis of the lead finger. There is, however, no provision for precisely locating the bond site on a lead finger an exact distance from the tip of the lead finger. Typically, the bonding tool is programmed by an operator to move a set distance away from the bond pad for placing the bond site along the longitudinal axis of a lead finger. This is not an accurate or consistent method for teaching a bond site location on a lead finger. In addition, this method of locating a bond site tends to use more bond wire than is necessary because the length of the bond wire is consistently oversized.
This situation is shown in FIG. 1. With reference to FIG. 1, a semiconductor die 10 is shown during a wire bonding process. The semiconductor die 10 has been attached to a die mounting paddle 12 of a leadframe 14. The semiconductor die 10 includes a plurality of bond pads 18 which connect to various integrated circuits formed on the die 10. The leadframe 14 includes an arrangement of lead fingers 16 which will ultimately become the external leads of the completed semiconductor package. During the wire bonding process, fine bond wires 26 are bonded to the bond pads 18 and to bond sites 20 on the lead fingers 16.
For teaching the location of a bond site 20' on a particular lead finger 16', the locations of the lateral edges 22', 24' of the lead finger 16' are sensed by the vision system of the wire bonding apparatus. The wire bonding apparatus is then programmed to determine a midpoint between the edges 22, 24 of the lead finger 16'. This is the dimension "Y" in FIG. 1. Accordingly, axis 28' is the longitudinal axis of the lead finger 16'. Following this determination, the bonding tool of the wire bonding apparatus is programmed to move along the longitudinal axis 28' of the lead finger 16', a predetermined distance "X" from the appropriate bond pad 18 on the semiconductor die 10, to locate the bond site 20' and make the bond.
A problem with this method of teaching or locating the bond site 20' on the lead finger 16' is that the location of the lead finger 16' with respect to the die 10 my vary from die to die. As an example, the location of the die 10 on the mounting paddle 12 may vary by up to 10 mils in any direction from the nominal position. The distance "X" is therefore imprecise. In addition, more bond wire than necessary tends to be used in order to insure that the bond site 20 will be located on the lead finger 16.
In view of these shortcomings of the prior art, it is an object of the present invention to provide an improved method for wire bonding semiconductor dice to the lead fingers of a leadframe and for teaching a bond site location on a lead finger during wire bonding. It is a further object of the present invention to provide an improved method for precisely locating a bond site along a longitudinal axis of a lead finger a predetermined distance from a terminal edge of a tip portion of the lead finger during wire bonding. It is a yet another object of the present invention to provide a wire bonding method in which a length of the bond wire is controlled during the wire bonding process and less bond wire is used. It is a further object of the present invention to provide an improved wire bonding method that is simple, efficient, and adaptable to large scale semiconductor manufacture.